Embodiments of the invention relate to polishing semiconductor substrates. More particularly, embodiments of the invention relate to pads to perform chemical-mechanical polishing on semiconductor substrates or layers formed on such substrates.
Integrated circuits are typically formed on substrates (for example silicon wafers) by the formation of various conductive, semiconductive and/or insulative layers. Although a layer may be etched after deposition to form a planarized surface, planarity may be improved with other techniques. Chemical mechanical polishing (CMP) is one accepted method of planarization, which tends to achieve improved planarity over an etch-back technique. CMP typically requires the substrate or wafer be mounted on a carrier or polishing head, with the surface of the layer to be polished exposed. The wafer is then placed against a moving (mechanical) polishing pad. The carrier head may rotate, as well as provide other motion between the wafer and the polishing surface. A polishing slurry, typically including an abrasive and at least one chemically reactive agent, may be introduced onto the polishing pad surface to provide the chemical component of CMP to polish the exposed surface.
When CMP is used on certain materials, such as a deposited copper layer of a semiconductor device, several problems may arise. First, copper generally has to be oxidized before a chemical slurry removes it. Second, dishing and erosion problems may occur between the abrasive particles in the slurry and copper, such as interconnects or lines on the wafer. To minimize dishing and erosion of a copper layer, polish pads have been manufactured with abrasive particles incorporated into the pad. However, these pads tend to introduce higher defects during polishing because the abrasives are fixed and, therefore, the friction is not a rolling friction. Pads with incorporated abrasive particles also may need special indexing mechanism and tooling to be implemented. Additionally, a continuous pad surface typically is desired because the abrasive is spent after a first pass. Another problem with using chemical slurry in CMP polishing of copper is that a copper containing waste stream may be generated and may present an environmental hazard that may require special handled.
A typical polishing pad includes a hard top layer and a softer bottom layer. The top layer has a high friction polishing surface, capable of transporting slurry, and the opposite surface adhesively bonded to a surface of the bottom layer. The opposite surface of the bottom layer is typically attached to a platen by a high strength pressure-sensitive adhesive layer.
Typically, a conventional polish pad serves at least two purposes: (1) It provides a mechanism to hold and transfer slurry and (2) it provides a frictional surface that helps in abrading the material being polished. Such conventional polish pads are generally made of cast polyurethane and non-woven impregnated polyurethane felts. Although these pads provide adequate CMP performance on various materials used in device fabrication, the existing pads have shortcomings in respect to polishing copper. Conventional polish pads and methods for CMP of copper suffer from the drawbacks of high dishing values, high defect values, poor margin for over polish, and the need for elaborate and expensive handling of the copper waste stream.